The present disclosure relates to a process mechanism for producing alkylaromatics, especially monoalkylaromatic compounds, for example ethylbenzene, cumene and sec-butylbenzene.
The alkylaromatic compounds ethylbenzene and cumene, for example, are valuable commodity chemicals which are used industrially for the production of styrene monomer and coproduction of phenol and acetone respectively. In fact, a common route for the production of phenol comprises a process which involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide, and then cleavage of the hydroperoxide to produce equal molar amounts of phenol and acetone. Ethylbenzene may be produced by a number of different chemical processes. One process which has achieved a significant degree of commercial success is the vapor phase alkylation of benzene with ethylene in the presence of a solid, acidic ZSM-5 zeolite catalyst. Examples of such ethylbenzene production processes are described in U.S. Pat. No. 3,751,504 (Keown), U.S. Pat. No. 4,547,605 (Kresge) and U.S. Pat. No. 4,016,218 (Haag).
Another process which has achieved significant commercial success is the liquid phase process for producing ethylbenzene from benzene and ethylene since it operates at a lower temperature than the vapor phase counterpart and hence tends to result in lower yields of by-products. For example, U.S. Pat. No. 4,891,458 (Innes) describes the liquid phase synthesis of ethylbenzene with zeolite Beta, whereas U.S. Pat. No. 5,334,795 (Chu) describes the use of MCM-22 in the liquid phase synthesis of ethylbenzene.
Cumene has for many years been produced commercially by the liquid phase alkylation of benzene with propylene over a Friedel-Crafts catalyst, particularly solid phosphoric acid or aluminum chloride. More recently, however, zeolite-based catalyst systems have been found to be more active and selective for propylation of benzene to cumene. For example, U.S. Pat. No. 4,992,606 (Kushnerick) describes the use of MCM-22 in the liquid phase alkylation of benzene with propylene.
Typically, the zeolite catalysts employed in hydrocarbon conversion processes, such as aromatics alkylation, are in the form of cylindrical extrudates. However, it is known from, for example, U.S. Pat. No. 3,966,644 (Gustafson) that shaped catalyst particles having a high surface to volume ratio, such as those having a polylobal cross-section, can produce improved results in processes which are diffusion limited, such as the hydrogenation of reside.
Moreover, it is known from U.S. Pat. No. 4,441,990 (Huang) that a polylobal catalyst particle having a non-cylindrical centrally located aperture can reduce the diffusion path for reagents and the pressure drop across packed catalyst beds while minimizing catalyst loss due to breakage, abrasion and crushing. In particular, Example 8 of the '990 patent discloses that hollow trilobal and quadrulobal ZSM-5 catalysts are more active and selective for the ethylation of benzene at 410° C. and 2169 kPa-a (kilopascal absolute) pressure than solid cylindrical catalysts of the same length. Under these conditions, the reagents are necessarily in the vapor phase.
Current commercial catalysts used most often for these process mechanisms are 0.159 cm cylindrical or 0.127 cm quadrulobal extrudates. The prior extrudates are roughly 1550 to 1600 microns in size, and the latter are roughly 1250 to 1300 microns in size.
Existing alkylation processes for producing alkylaromatic compounds, for example ethylbenzene and cumene, inherently produce polyalkylated species as well as the desired monoalkylated product. It is therefore normal to transalkylate the polyalkylated species with additional aromatic feed, for example benzene, to produce additional monoalkylated product, for example ethylbenzene or cumene, either by recycling the polyalkylated species to the alkylation reactor or, more frequently, by feeding the polyalkylated species to a separate transalkylation reactor. Examples of catalysts which have been used in the alkylation of aromatic species, such as alkylation of benzene with ethylene or propylene, and in the transalkylation of polyalkylated species, such as polyethylbenzenes and polyisopropylbenzenes, are listed in U.S. Pat. No. 5,557,024 (Cheng) and include MCM-49, MCM-22, PSH-3, SSZ-25, zeolite X, zeolite Y, zeolite Beta, acid dealuminized mordenite and TEA-mordenite. Transalkylation over a small crystal (<0.5 micron) form of TEA-mordenite is also disclosed in U.S. Pat. No. 6,984,764 (Roth et al).
Where the alkylation step is performed in the liquid phase, it is also desirable to conduct the transalkylation step under liquid phase conditions. However, by operating at relatively low temperatures, liquid phase processes impose increased requirements on the catalyst, particularly in the transalkylation step where the bulky polyalkylated species must be converted to additional monoalkylated product without producing unwanted by-products. This has proven to be a significant problem in the case of cumene production where existing catalysts have either lacked the desired activity or have resulted in the production of significant quantities of by-products such as ethylbenzene and n-propylbenzene.
U.S. Pat. No. 6,888,037 (Dandekar et al) discloses a process for producing cumene which comprises the step of contacting benzene and propylene under at least partial liquid phase alkylating conditions with a particulate molecular sieve alkylation catalyst, wherein the particles of said alkylation catalyst have a surface area to volume ratio of about 80 to less than 200 inch−1. According to U.S. Pat. No. 6,888,037, the liquid phase propylation of benzene, unlike the liquid phase ethylation of benzene, is sensitive to intraparticle (macroporous) diffusion limitations. In particular, by selecting the shape and size of the particles of the alkylation catalyst such that the surface to volume ratio is within the specified range, the intraparticle diffusion distance can be decreased without excessively increasing the pressure drop across the first catalyst bed. As a result, the activity of the catalyst for the propylation of benzene can be increased, while at the same time the selectivity of the catalyst towards undesirable polyalkylated species, such as diisopropylbenzene (DIPB) can be reduced.
U.S. Patent Application Ser. No. 60/808,192 discloses a process for producing a monoalkylated aromatic compound in an alkylation reaction zone, said process comprising the steps of (1) providing said alkylation reaction zone with an alkylatable aromatic compound, an alkylating agent, and a catalytic particulate material; and (2) contacting said alkylatable aromatic compound and said alkylating agent with said catalytic particulate material in said alkylation reaction zone maintained under alkylation conditions, to form a product comprised of said monoalkylated aromatic compound and polyalkylated aromatic compound(s), wherein the majority of said catalytic particulate material has a surface area to volume ratio of greater than about 79 cm−1.
According to the present disclosure, it has now unexpectedly been found that the reaction of the present disclosure conducted in the presence of a specific catalyst manufactured from extrudate to comprise catalytic particulate material within the narrow range of from about 125 microns to about 790 microns in size and having an Effectiveness Factor, hereafter defined, increased from about 25% to about 750% from that of the original extrudate, yields a unique combination of activity and selectivity while not subjecting the process to unacceptable pressure drop across the catalyst bed. This is especially the case when the process involves liquid phase alkylation for manufacture of monoalkylated product, particularly for the liquid phase alkylation of benzene to ethylbenzene or cumene. This obviates the demand in many instances for the difficult transalkylation reaction for conversion of unwanted bulky polyalkylated species in such a process.